Traditionally a GSM Public Land Mobile Communication Network is implemented using a core network, including core network nodes such as Mobile Services Switching Centers providing e.g. circuit switched services and Short Message Services, and a radio network, referred to as a GSM Base Station System (BSS), providing radio coverage for the Public Land Mobile Network. Each GSM Base Station System includes one or more Base Station Controllers each connected to and controlling a plurality of base transceiver stations (base stations) which each includes radio transmitter and receivers (transceivers) for providing radio communication in one or more cells.
More recently, radio network sharing has been implemented as a means to allow network operators reducing the deployment costs for cellular networks. In a scenario with a shared GSM radio network, i.e. a shared GSM base station system, the GSM base station system would provide radio coverage for two or more GSM Public Land Mobile Networks by providing radio communication in cells belonging to different GSM Public Land Mobile Networks. All cells belonging to a certain GSM Public Land Mobile Network can be regarded as a logical radio network for said network. This way, all but the resources dedicated to specific cells (each belonging to a single network), can be shared between said one or more GSM Public Land Mobile Networks.
Handover is a well known feature of cellular communication networks enabling a connection supported by radio resources (one or more channels) in a serving cell to be handed over to a new cell where radio resources are allocated for continued support of the connection. Different types of handovers can be distinguished based on what nodes are involved in the handover. Thus there are intra BSC handovers between cells for which the same base station controller handles resource allocation, inter BSC handover between cells where different base station controllers handle resource allocation in the serving and new cells and there is also inter PLMN handovers between cells belonging to different Public Land Mobile Networks. For inter BSC handovers and inter PLMN handovers, the base station controller of the serving cell needs to notify the core network (mobile services switching centre) that it would be desirable to perform handover and await confirmation that the handover may be performed prior to handover.
Handover is typically performed to enable continued communication as a mobile station moves from the coverage area of a serving cell into the coverage area of another cell. However, many GSM cellular network support Cell Load Sharing (CLS, see e.g. U.S. Pat. No. 6,266,531) where handover may be performed to distribute traffic load between cells. Thus, when a base station controller detects too high traffic load in a first cell, while there is spare capacity available in neighboring cells, the base station controller may order handover of connections in the first cell to neighboring cells having capacity for handling additional load and radio resources capable of supporting said connections. Cell Load Sharing is performed between cells belonging to the same Public Land Mobile Network and for which the same base station controller is responsible for resource allocation.